In most wireless systems, e.g. GSM (Global System for Mobile communications), WCDMA (Wideband Code Division Multiple Access), WLAN (Wireless Local Area Network), special well known training sequences or pilot signals are transmitted so that the receiver can estimate the channel parameters sufficiently well for detection of any data signal, not previously known by the receiver. Several methods exist to do this, some use user specific pilots and some use common pilots or combinations. Some pilots are code spread and overlaid with user data, others have dedicated time-frequency slots when pilots are transmitted. In any case, some part of the available radio resources must be allocated for pilots resulting in overhead that cannot be used for data.
In single-carrier systems, such as e.g. described in U.S. Pat. No. 6,452,936, pilot data can be provided in certain time slots within a transmission frame. A shorter time interval between successive pilot data gives a more accurate channel estimation, but decreases instead the transmission rate. In U.S. Pat. No. 6,452,936, a particular code of the CDMA system is allocated to a user. A pilot density of a frame structure is continuously selected dependent on channel estimation information.
A multi-carrier approach has been proposed in wireless communications systems, in which a data stream typically is separated into a series of parallel data streams, each of which is modulated and simultaneously transmitted with a different frequency. An example of a multi-carrier system is an OFDM (Orthogonal Frequency Division Multiplexing) system. This allows a relative size of transmitted symbols relative to a multipath delay to be much larger which reduces intersymbol interference. Such a cellular multi-user, multi-carrier wireless communications system thus allows a particular user to utilise more than one carrier simultaneously. The allocation of one or several carriers depends typically on quality of service consideration, such as requested transmission rate. Generally, in a multi-carrier, multi-user system, the resource space is used in a flexible manner to give each user the best possible quality at each time. The principles and requirements for providing channel estimations become in this way more complex than in a single-carrier system, since a continuously use of a single communication resource is not ensured. In a cellular multi-user, multi-carrier wireless communications system, the base station must accommodate many users that each experiences different channel characteristics due to fading in both time and frequency. Furthermore, different users travel at different speeds and thus experience different Doppler shifts.
Today, there are a few multi-carrier systems in use. However, they are not particularly designed for the difficult, ever changing, hard-to-predict multi-user environments that are envisioned for future wireless systems.
For example, the systems for DVB/DAB (Digital Video Broadcasting/Digital Audio Broadcasting) are broadcast systems that cannot take into account the need for individual users. Such systems must design their pilot structure according to the worst-case scenario so that detection becomes possible even under the worst possible conditions. Such a pilot structure gives rise to a substantial pilot overhead, and is indeed necessary in these worst-case scenarios. However, whenever the situation is better than the worst case, which typically is the case most of the time, the pilot structure is unnecessarily extensive, giving an unnecessary pilot overhead for most users. The pilot overhead can indeed be substantial. This reduces data capacity in the own cell and furthermore increases the interference to the neighbouring cells (so called ‘pilot pollution’).
Another example of a multi-carrier system is WLAN (i.e. IEEE 802.11a, IEEE 802.11g). Such a system is designed for a limited geographical area in which the users are stationary or slowly moving. The design is not intended for conditions in which the user is moving quickly or for handling mobility in a multi-cellular environment.
In the published US patent application 2003/0215021, a communications system is disclosed, in which channel characteristics are determined by analysing a signal received over a (sub)-carrier. The determined characteristics are then used to divide the sub-carriers into groups of similar fading characteristics. Each group is then allocated a pilot sub-carrier. The determined pilot allocation scheme is then used for future transmissions across the sub-carrier. This system compensates for differences in fading characteristics over the carrier bandwidth, but has a disadvantage in that it is assumed that a sub-carrier is continuously used for one single user. A user has to have access to a large number of sub-carriers in order to make such a pilot allocation efficient. Furthermore, entire sub-carriers are allocated as pilot sub-carriers, which occupies a large part of the available resource space, contributing to the pilot pollution.